Business leaders have also issued a statement calling on the court to annul the vote, paving the way for early elections.

The influential TUSIAD association said a vote was necessary "to preserve the inseparable principles of democracy and secularism".

Mr Gul has steered Turkey's European Union accession talks as foreign minister and is seen as less confrontational than Prime Minister Recep Tayyip Erdogan, the leader of his Islamist-rooted Justice and Development (AK) party.

"The president must be loyal to secular principles. If I am elected, I will act accordingly," he said after his nomination for the presidency.

But some analysts say he is closer to his religious roots, and his wife would be the first First Lady to wear a headscarf, a deeply divisive statement in Turkey.

'Test case'

The BBC's Sarah Rainsford says secularists are concerned that if he is elected, the AK party will control the presidency, the government and parliament.